The present invention relates to a method of forming interconnections and more specifically to a method via which the surfaces of aluminum type material connectors and junctions can be protected from detrimental influences by oxygen and moisture in the surrounding environment immediately following etching, and thus enable the fabrication of highly reliable interconnectors in a multi-chamber fabrication device without incurring increases in production costs.
With the recent advent of VLSO, ULSI the demand for high accumulation and high performance semiconductor devices has induced the situation wherein, during the fabrication of the interconnector patterns, the chances for the various layers and surfaces of interconnector patterns to come into contact with the ambient atmosphere or with atmospheres which contain acidic substances and undergo chemical changes, has increased. One of these chemical changes takes the form of the surfaces of Al type material interconnector patterns, which are exposed immediately following the etching process which forms the same, to undergo so called "after corrosion" or for the exposed surfaces of contact and bore holes to develop a natural oxide film thereon.
The above mentioned after corrosion is mainly brought by corrosion which occurs after dry etching of aluminum type interconnector layers. An example of the mechanism involved is set forth on pages 101-106 of the monthly SEMICONDUCTOR WORLD of April 1989.
The dry etching of Al type interconnector layers is usually carried out using chlorine containing gases. For example, JP-B-59-22374 discloses the use of a gas containing a mixture of BCl.sub.3 /Cl.sub.2. The effect of this usage is such that reaction products such as AlCl.sub.x and those which result from the decomposition of the etching gas become adsorbed at locations which are proximate the etching pattern. In addition to this adsorption, such products can be absorbed into the resist mask.
These chlorine based reaction products and decomposed etching gas are absorbed by air-borne moisture to form droplets of electrolyte which give rise to corrosion and eat away the aluminum. Further, the resist mask and the active types of chlorine react to form a carbon based polymer CCl.sub.x which forms an anisotropic protective side wall layer. However, in this instance the chlorine in the carbon based polymer acts as a source of harmful residual chlorine.
In connection with the above mentioned after corrosion, a measure against the stress migration in the Al based interconnector materials has been to add Cu (copper). However, this only results in worsening matters. That is to say, CuCl becomes one of the etching reaction products which also remains proximate the etched pattern due to the low vapor pressure which prevails after the etching. This CuCl is absorbed by the moisture and functions to provide chloride ions (electrolyte) while the aluminum and copper act as the electrodes of a battery cell.
In addition, during recent years, stringent semiconductor device design rules along with looking to the prevention of after corrosion, have amounted to factors which have brought about a reduction in the sole use Al based interconnector layers. By way of example, in order to prevent the separation of Al based interconnector layer and the silicon substrate, it is common to provide a barrier metal.
Further, in order to improve the precision of photolithography on the surfaces of the Al based interconnector layers, an amorphous silicon or TiON reflection preventing layer has been laminated. In this instance, as the etching of the layers of different contacting materials, exposes edge surfaces of the same, droplets which are formed in the atmospheric air, of course tend to provide a battery cell effect which tends to hasten the rate with which the aluminum is eaten away. Further, chlorine and chlorine containing materials can be enter and remain in micro sized cracks and the like in the different type material layers.
The mechanism via which the above mentioned after corrosion takes place is such that with the exception of fluorine, halogens such as bromine etc., provide essentially the same effect as chlorine and will be referred to generically in the instant disclosure simply as halogens.
In order to prevent after corrosion it is known to (a) use CF.sub.4 and CHF.sub.3 fluorocarbon gas for plasma cleaning, (b) use oxygen plasma ashing for the removal of side wall protective layers and resist masks, and (c) use NH.sub.3 gas for plasma cleaning in combination with washing techniques.
Each of these measures is directed to the removal of residual halogens. That is to say, the reaction products which are produced when chlorine and bromine are replaced with fluorine, increases the vapor pressure and the residual halogens which are mainly included in the resist mask or side wall protective layers are removed by ashing; or the halogen containing materials are converted into inactive halogen containing materials such as ammonium halogens, or alternatively highly anti-corrosive layers of AlF.sub.3 or Al.sub.2 O.sub.2 are formed on the surfaces of the Al based interconnector layers, and thus attenuate after corrosion.
Nevertheless, none of the above provides a decisive effect.
On the other hand, a different approach to the removal of the residual halogens comes in that, after patterning the Al based interconnector layer, CHF.sub.3 or the like gas is used to accumulate a carbon based polymer layer over the surface of the wafer. This is referred to as polymer passivation. By way of example, the 36th Applied Physics related League lecture meeting (1989 Annual Spring meeting) Volume 2 page 571, Subject No. 1p-L-4 and SEMICONDUCTOR WORLD October 1990 pages 44-49 (Press Journal Publishing Co.) it is disclosed that after the Al based interconnector layer etching is completed, immediately before over etching, via CHF.sub.3 plasma deposition and etchback, the side walls of the Al based interconnector pattern are selectively covered with a side wall protective coating of carbon based polymer.
Originally, over etching was proposed as a technique for preventing inferior anisotropic formation, however its annexing effect also attenuates after corrosion. The fluorine from the CHF.sub.3 substitutes with the residual halogen on the wafer or forms a carbon based polymer which prevents moisture from reaching the Al based interconnector layer, so that until the next fabrication step it is possible to extend the time before the next fabrication step need be carried out.
On the other hand, in the case of natural oxide films, in recent years the use of multi-interconnector layers in semi-conductor device has lead a cross-up problem. As is commonly known, high quality complex semiconductor devices increase the area consumed by the interconnectors, and in order to prevent the size of the chip being increased it is necessary to use multiple interconnector layers.
In order to achieve the above mentioned multiple interconnector layers it is necessary to form electrical connections between the layers by etching contact and bore holes which extend between the various levels. After removing the resist mask used for the etching the interconnector the holes are filled as the interconnector layers are formed. In this process the wafer is transferred from the etching chamber to a ashing device which removes the resist mask. However, during the transfer the wafer is briefly exposed to the atmospheric air. Following the removal of the resist mask, the wafer is transferred to another process chamber (e.g. a sputtering chamber) and is again exposed to the atmospheric air.
During the period the wafer is exposed to the atmospheric air, the surfaces of the connector layers at the bottom of the connection holes are exposed to air and inevitably tend to form natural oxide films thereon. The natural oxide films cause the resistance of the contacts to increase.
During recent years as the aspect ratio of the contact holes has increased tungsten type metals have been selectively developed via CVD. However, the natural oxide films have either obstructed or induced abnormal development. Accordingly, it is essential that these oxide films be completely removed.
Previously, the so called wet method wherein a mist of HF buffer has been used to moisturize the surface of the substrate, has been widely used. However, this has then demanded washing and drying steps and has been troublesome. On the other hand, while the so called dry method wherein gaseous Hf is used, is being experimented with, various control problems have been encountered. Further, even if the natural oxide film is completely removed the problem that the wafer will again be exposed to air and the natural oxide film be reformed exists.
To overcome this latter mentioned problem various techniques have been investigated from various aspects including various hardware and process approaches.
First, in connection with the hardware approaches, a multi-chanber system has been proposed wherein a vacuum road-lock device has been developed and wherein a wafer handling unit is arranged to transfer the wafer between a plurality of surrounding process chambers. This wafer handling unit and each of the chambers are separated by a gate valve and thus arranged so that the wafer can be be moved from chamber to chamber without be exposed to atmospheric air. By way of example SEMICONDUCTOR WORLD Sept. 1990 pages 126-130 disclose a multichamber system in which tungsten interconnectors are formed.
On the other hand, in connection with the process approach, so called polymer passivation has been proposed wherein a carbon based polymer is formed over the interconnector surface to shut out any contact with atmospheric air. An example of this is set forth in 34th Applied Physics related League lecture meeting (1987 Annual Spring meeting) Volume 2 page 460 Subject No. 31p-P-6. In this example, after the natural oxide film is removed a carbon based polymer is induced to accumulate on the surface of the silicon substrate. Immediately before the next fabrication process, the polymer layer is removed using chlorine in an UV assisted etching.
However, with the above mentioned prior art approaches various problems have been encountered.
Firstly, in connection with the measures used to overcome after corrosion there are certain points which are not covered. Among these is the problem that when the carbon based polymer does not adequately provide a step coverage there are sites where moisture can penetrate. Further, when the resist mask and the carbon based polymer layer are both present, while a very effective protection is achieved, a problem is encountered in that during ashing a high temperature is applied, the formation of polymer requires the application of an incompatible low temperature which results in the optimal conditions being extremely difficult to achieve and give rise to the concern that polymer particle contamination might result. Furthermore, in the case the polymer is formed after the resist mask, the process is such that, in order to remove the polymer, the ashing procedure must be carried out a number of times and gives rise to the problem that the number of operations is increased.
In the case of the removal of the natural oxide film, while the use of multichamber system permits the various steps to be stably carried out in the appropriate atmosphere, and for the number of steps and chances of pollution to be minimized, the actual device itself is extremely large and in order to achieve the required high level processing, the running costs are high.
In addition to the above, especially in the instance the interconnector layer takes the form a doped silicon substrate, the use of a carbon based polymer induces the fear that carbon contamination will occur.